The invention relates to a method of applying a silver layer to a glass substrate, which silver layer comprises a first and a second silver layer, in which method the substrate is provided, after it has been activated, with the first silver layer from an electroless silver bath, whereafter the first silver layer is provided with the second silver layer by means of an electroplating process.
Examples of glass substrates are, in particular, glass plates for use in plasma displays, cathode-ray displays, field-emission displays and thin electron displays in which holes are formed to transport electrons. Said glass plates may be constructed as plates and provided with addressable electrodes of silver. Silver is preferred because it has the highest electric conductivity of all metals. This is necessary because of the small line widths of the electrode patterns of, typically, 100 .mu.m and thicknesses of approximately 2 .mu.m.
Such a method is disclosed, inter alia, in "Electroplating of Plastics, Handbook of theory and practice, Finishing Publications Ltd., Hampton Hill, GB, pp. 8-9. Said handbook states that, for example, glass can be provided with a silver layer by means of a wet-chemical process, which silver layer is obtained by chemical reduction. Owing to the low deposition rate of electroless baths, the thin silver layer is subsequently reinforced with a thicker second silver layer which is formed by electrolysis (electroplating). As described in said handbook, there is a risk that in the case of relatively thick silver layers, the internal stresses in the silver layer increase to such an extent that the silver layer and the substrate become detached. The adhesion between the silver layer and a glass substrate can be improved by mechanically roughening the substrate by means of powder blasting with, for example, aluminium-oxide particles. The resultant hair cracks in the glass surface are subsequently etched selectively with, in succession, an aqueous HF and HNO.sub.3 solution, which results in deeper hair cracks. The silver layer to be provided is mechanically anchored to the substrate surface and hence adheres well thereto.
The above-described method has a number of drawbacks. Powder blasting results in a quantity of arsenical chemical waste, which originates from the (borosilicate) glasses used. After use, the HF solutions employed form hazardous chemical waste. The mechanical strength of the glass plates having a typical thickness of only 0.4 mm is reduced considerably. Moreover, three additional process steps (powder blasting, HF-etching and HNO.sub.3 -etching) are necessary to make the silver adhere properly.